This invention relates to the simple fabrication of removable electronic interface modules for connecting fiber optic communication or signal lines to a computer and, more specifically, to the chassis structure which permits efficient and reliable assembly of both the interfacing module electronic components and the shielding of the module to prevent, to the maximum extent possible, electromagnetic radiation from escaping the module.
Computers increasingly are being connected to communications lines and other devices or networks with the computers performing as servers to the peripherally connected computers or devices. The volume of data sent and received by the computer serving as a server of a network is such that the networks are advantageously constructed using fiber optic lines in order to increase the throughput of data.
Fiber optic lines and the associated fiber optic signals require transceivers to convert the optical light pulse signals to electronic signals which are usable by the computer. Such a transceiver includes a transmitter optical subassembly and a receiver optical subassembly to send and receive the optical signals.
Industry standards have been established to define the physical parameters of the said devices and, particularly, the overall interface. This permits the interconnection of different devices manufactured by different manufacturers without the use of physical adapters.
Since about 1990, the fiber optic industry has been using a so-called xe2x80x9cSC duplex fiber optic connector systemxe2x80x9d as the optical fiber connector interface on the front of fiber optic transceivers. The physical separation between the transmitter optical subassembly and receiver optical subassembly (TOSA and ROSA, respectively) for the SC duplex connector is approximately 12.7 mm. However, the industry is now converting to the so-called xe2x80x9cSmall Form Factor optical connectorsxe2x80x9d and associated xe2x80x9cSmall Form Factor optical transceiver.xe2x80x9d In the so-called Small Form Factor optical connectors, the separation between the transmitter optical subassembly and receiver optical subassembly is established at approximately 6.25 mm, less than half the separation of the prior SC duplex connector. The Small Form Factor (SFF) standard establishes a module enclosure, having a 9.8 mm height and a width of 13.5 mm and allows a minimum of 24 transceivers arranged across a standard rack opening. The reduction in size from the former SC duplex connector standard to the Small Form Factor standard requires both substantial redevelopment and redesign.
Moreover, the Small Form Factor optical fiber connector interface has been adopted as a standardized removable module. The module may be connected to a module interface on the host circuit board of a computer. The transmitter optical subassembly/receiver optical subassembly in the module of the Small Form Factor optical fiber connector interface, the processor, and all long conductors of the transmitter optical subassembly and receiver optical subassembly, individually and collectively, radiate electromagnetic radiation and create electromagnetic interference for other electronic devices and components which are exposed to the electromagnetic radiation.
The use of a separate removable module containing a variety of electronic devices requires that the module be easily and inexpensively assembled and that the module further provide its own electromagnetic radiation shielding. Due to the size constraints placed on the interface modules, the shielding must be small but at the same time efficient in collection and grounding of the collected electromagnetic radiation.
It is an object of the invention to simplify assembly of a modular interface for receiving and sending optical data signals.
It is another object of the invention to provide a chassis for an optical interface for data signals whereby the structural characteristics of the chassis permit assembly at precise locations for the electronic components and the shielding permits both accurate connection to the host circuit board and proper alignment of the interface optical subassemblies with the connected optical fibers.
It is a further object of the invention to provide reliable assembly of a plurality of parts and components into a module for translation of the data signals between electrical and optical form.
It is still another object of the intention to make the module easily removable from the host device as well as effectively suppress electromagnetic radiation to the greatest extent possible.
A module for interfacing communication line or lines to the main system of the computer is designed to be removable. In order to provide the removability factor, the module must contain sufficient electronic circuitry to convert signals between optical fiber conveyed light pulses and electrical digital signals or vice-versa. This conversion requires at least a laser driver, post amplifier, and supporting circuitry as well as transmitter optical subassembly and a receiver optical subassembly which comprise light generating and light sensitive electronic devices, respectively. The operation of the electronics causes radiation of electromagnetic energy which may cause interference, if not suppressed, with nearby electromagnetic radiation sensitive equipment and components.
An easily removable module is advantageously assembled on a chassis which is molded plastic, preferably, selected for its durability and insulative characteristics. The chassis has an open channel structure for a portion of its length, and a duplex port receiving end for accepting fiber optic conductor connectors. The chassis is formed to include positioning surfaces against which an electronic circuit board is positioned, and a pair of locating pins or projections define the position of the electronic circuit board relative to the chassis.
In areas lacking additional stabilization, the walls of the chassis are formed thinly enough that the walls may be deflected to permit snap-in positioning of the circuit board within the chassis and allow retention latches to be forced out of the path of a circuit board once inserted into the chassis.
Once assembled, the chassis then is substantially enclosed with shielding to suppress the escape of electromagnetic radiation. The shield is fabricated of electrically conducting sheet metal, such as thin metal plate stock, and formed into a channel shape. The channel shaped shield is provided with edge tabs. Each edge tab may be bent to form spring contacts which contact and electrically ground to a bracket or frame member; additionally, tabs may be bent over walls of the chassis to retain the shielding in its desired position. A separate shielding member is positionable on the chassis and retainable by bending or crimping of the edge tabs as contact between the separate shield and the edge tabs establishes grounding electrical contact therebetween. The emissions of electromagnetic radiation from the duplex port end of the chassis are reduced and modified by extending a shield member between the transmitter optical subassembly/receiver optical subassembly, bridging the separate shield and the channel shaped shield to reduce the effective opening to attenuate the electromagnetic radiation passing through the duplex port end of the chassis.
However, it is known that an aperture will attenuate electromagnetic radiation waves when the aperture is less than xc2xd of the wavelength xcex to be attenuated (i.e., length of aperture  greater than xc2xdxcex). Moreover, the smaller the aperture, the greater the attenuation of the electromagnetic radiation waves. The attenuation of electromagnetic radiation waves due to passage through an aperture can be determined using the following formula:
S=20 log(xcex/2L), where
S=the shielding effectiveness of the aperture (in decibels);
xcex=the wavelength of the electromagnetic radiation; and
L=the maximum linear length of the aperture (in meters).
Moreover, the wavelength xcex can be determined by dividing the velocity of the electromagnetic radiation wave (i.e., the wave speed, which is approximately 3xc3x97108 m/sec2) by the frequency of the electromagnetic radiation emissions.
Thus as the operational frequency (and hence, speed) of an electrical device increases, the associated wavelengths become smaller, thus requiring smaller apertures. However, the apertures are typically limited in minimum size to allow for the passage of associated cables or connections, for example. If the size of the apertures are reduced too much, then passage of the associated cables or connections therethrough may be prohibited. Therefore, it is clear that the size of the aperture, which typically has a minimum size, may limit the speed (i.e., bandwidth) of the associated electrical device.